Recently, as wireless LAN systems become faster and less expensive, they are now in significantly increasing demand. Particularly, in these days, the introduction of personal area networks (PAN) has been examined for performing information communications by constructing a small-scale wireless network between a plurality of electronic apparatuses around a person. For example, various wireless communications systems using frequency bands including 2.4 GHz band, 5 GHz band, and so forth, and requiring no licenses issued by oversight authorities have been established.
In the case of the wireless communications using the wireless LAN or the like, information is transmitted via an antenna. For example, various kinds of unbalanced antennas are in practical use. As a rule, the unbalanced antennas have a radiation conductor and a ground conductor that are provided with a predetermined gap therebetween. An electric signal is fed to the gap. In general, the electric signal is fed from the rear side of the ground conductor. In this case, a hole is bored in the ground conductor and the radiation conductor is extended toward the rear side.
Example shapes of the radiation conductor are shown in FIG. 1 illustrating a monopole antenna, FIG. 2 illustrating a helical antenna, FIG. 3 illustrating a plate-like monopole antenna, and FIG. 4 illustrating a monoconical antenna.
As a relative merit of the unbalanced antenna versus a balance antenna, the unbalanced antenna can be directly connected to a coaxial transmission line used as a line for feeding an electric signal. In general, the coaxial transmission line is highly resistant to an external noise. That is to say, a coaxial cable basically functions as an unbalance cable that can function in keeping with the unbalanced antenna. On the other hand, where the balance antenna is used, a balance-to-unbalance converter is needed between the balance antenna and the coaxial cable. Further, since the ground conductor can be used with a case ground conductor of the device or provided so as to be in intimate contact therewith, the device can be downsized, which is advantageous for mounting.
In general, the ground conductor has a disk shape measuring at least a half wave or so in diameter. However, it is often difficult to achieve the size for mounting the ground conductor on a small wireless apparatus. A significantly small ground conductor deteriorates its reception characteristic or the like, thereby affecting the operation of the antenna.
The deterioration of the unbalanced antenna's characteristic due to the downsized ground electrode will now be described below. Here, calculations are performed for studying the characteristic change caused by significantly reducing the size of a disk-shaped ground conductor measuring a half wave in diameter by referring to a disk monopole antenna shown in FIG. 5, as an example. An electric signal is fed via the coaxial transmission line from the rear side of the ground electrode. The conditions for calculating the antenna characteristic are shown below.
1. Radiation Conductor
a metal having a conductivity of 1×107 S/m
24.8 mm in diameter, 0.8 mm in thickness
2. Ground Conductor
a metal having a conductivity of 1×107 S/m
reduced from a disk being 50 mm in diameter and 0.8 mm in thickness to a rectangular plate being 24.8×4×0.8 mm (reduced by 5 percent in area ratio)
3. Feed Section
a gap of 0.8 mm
a coaxial transmission line having a characteristic impedance of 50 Ω
FIG. 6 illustrates the calculation result of a characteristic of the disk monopole antenna having the disk-like ground conductor measuring a half wave in diameter. In this drawing, the VSWR (Voltage Standing Wave Ratio) characteristic is shown on the left side, the radiation directivity in a vertical surface at 3 GHz is shown in the middle, and the surface-current density distribution also at 3 GHz (the density is shown by concentration) is shown on the right side.
As shown in this drawing, the VSWR value of about 2 or less is achieved over the range from 3.5 to 9 GHz. That is to say, a suitable impedance matching characteristic can be obtained over an ultra-wide band. Further, since the radiation directivity in the vertical surface at 3 GHz forms an 8-shape having peaks substantially along a horizontal direction, this disk monopole antenna has a characteristic similar to the inherent characteristic thereof (In a floor-limit frequency band, this antenna has a characteristic same as that of a dipole antenna.). According to the surface-current density distribution at this time, the level of an unnecessary leakage current flowing on an external conductor of the coaxial transmission line is low (Where the ground conductor has an infinite width, no leakage currents flow on the external conductor of the feed transmission line on the rear side.). Therefore, this calculation result of the radiation directivity is acceptable.
FIG. 7 illustrates the calculation result of a characteristic of the disk monopole antenna, where the ground conductor is reduced in size. As is the case with FIG. 6, the VSWR characteristic is shown on the left side, the radiation directivity in a vertical surface is shown in the middle, and the surface-current density distribution is shown on the right side.
A comparison between the characteristic shown in FIG. 7 and that shown in FIG. 6 shows a deterioration of the impedance-matching characteristic. The VSWR at from 3.5 to 9 GHz increases up to 3. The radiation directivity in the vertical surface at 3 GHz points downward in the extreme and drops to around −10 dBi in a horizontal direction. According to the surface-current density distribution at this time, a large leakage current flows on the external conductor of the coaxial transmission line and a radiation element from this leakage current affects the inherent radiation directivity. That is to say, the radiation directivity changes according to how the feed line is wired. In some cases, the above-described disturbances in the radiation directivity can cause a significant problem.
In summary, where the unbalanced antenna is mounted on the small wireless communications device and the ground conductor is reduced in size, it becomes impossible to make the most of the inherent characteristic of the antenna.